IssueTracker

Is there a full example you can link to which shows a shared element transition working with predictive back?

This one uses Navigation, but the guidance is the same.

Click to go to the second fragment, and dragging back will show the sharedElement box seeking.

Thank you for the sample and the insight it provides. I added a MaterialToolbar to the sample project and now some unexpected behaviour is encountered when I perform the back gesture before the animation finished playing. The back arrow and the second fragment label are still displayed in the MaterialToolbar. I am unsure if this is related to the Navigation library or the Fragment library.

Can the above sample please be triaged? There is definitely a bug with shared element transitions in combination with the navigation, but I am unable to tell where exactly things go wrong.

Re comment#5 - If you are having a problem only after adding a MaterialToolbar, you should file an issue on the Material Components.

The toolbar was for demonstration purposes to visualize the issue and is not causing the actual issue. When downgrading to the latest stable release of the jetpack Fragment library 1.6.2, the issue is resolved but shared element transitions are not supported in this version. Please try the sample I shared and experience it yourself. It definitely needs to be resolved before the stable release of Fragment 1.7.0

I added leakcanary and I think it reveals part of the underlying issue:

┬───
│ GC Root: Thread object
│
├─ java.lang.Thread instance
│    Leaking: NO (the main thread always runs)
│    Thread name: 'main'
│    ↓ Thread.threadLocals
│             ~~~~~~~~~~~~
├─ java.lang.ThreadLocal$ThreadLocalMap instance
│    Leaking: UNKNOWN
│    Retaining 710,0 kB in 1495 objects
│    ↓ ThreadLocal$ThreadLocalMap.table
│                                 ~~~~~
├─ java.lang.ThreadLocal$ThreadLocalMap$Entry[] array
│    Leaking: UNKNOWN
│    Retaining 710,0 kB in 1494 objects
│    ↓ ThreadLocal$ThreadLocalMap$Entry[10]
│                                      ~~~~
├─ java.lang.ThreadLocal$ThreadLocalMap$Entry instance
│    Leaking: UNKNOWN
│    Retaining 695,6 kB in 1424 objects
│    ↓ ThreadLocal$ThreadLocalMap$Entry.value
│                                       ~~~~~
├─ androidx.collection.ArrayMap instance
│    Leaking: UNKNOWN
│    Retaining 695,6 kB in 1423 objects
│    ↓ SimpleArrayMap.mArray
│                     ~~~~~~
├─ java.lang.Object[] array
│    Leaking: UNKNOWN
│    Retaining 695,5 kB in 1421 objects
│    ↓ Object[1]
│            ~~~
├─ androidx.transition.Transition$AnimationInfo instance
│    Leaking: UNKNOWN
│    Retaining 32 B in 1 objects
│    ↓ Transition$AnimationInfo.mTransition
│                               ~~~~~~~~~~~
├─ androidx.transition.Fade instance
│    Leaking: UNKNOWN
│    Retaining 54,1 kB in 1019 objects
│    ↓ Transition.mParent
│                 ~~~~~~~
├─ androidx.transition.TransitionSet instance
│    Leaking: UNKNOWN
│    Retaining 9,8 kB in 264 objects
│    ↓ Transition.mParent
│                 ~~~~~~~
├─ androidx.transition.TransitionSet instance
│    Leaking: UNKNOWN
│    Retaining 8,9 kB in 242 objects
│    ↓ Transition.mParent
│                 ~~~~~~~
├─ androidx.transition.TransitionSet instance
│    Leaking: UNKNOWN
│    Retaining 8,0 kB in 220 objects
│    ↓ Transition.mEndValuesList
│                 ~~~~~~~~~~~~~~
├─ java.util.ArrayList instance
│    Leaking: UNKNOWN
│    Retaining 60 B in 2 objects
│    ↓ ArrayList[4]
│               ~~~
├─ androidx.transition.TransitionValues instance
│    Leaking: UNKNOWN
│    Retaining 120 B in 4 objects
│    ↓ TransitionValues.view
│                       ~~~~
╰→ androidx.constraintlayout.widget.ConstraintLayout instance
​     Leaking: YES (ObjectWatcher was watching this because com.example.
​     pbfragmentsharedelement.FirstFragment received Fragment#onDestroyView()
​     callback (references to its views should be cleared to prevent leaks))
​     Retaining 2,6 kB in 55 objects
​     key = ecd720aa-87ad-4a48-b51f-d2a2884cd619
​     watchDurationMillis = 12386
​     retainedDurationMillis = 7385
​     View not part of a window view hierarchy
​     View.mAttachInfo is null (view detached)
​     View.mID = R.id.linearLayout
​     View.mWindowAttachCount = 1
​     mContext instance of com.example.pbfragmentsharedelement.MainActivity
​     with mDestroyed = false

METADATA

Build.VERSION.SDK_INT: 34
Build.MANUFACTURER: Google
LeakCanary version: 2.9
App process name: com.example.pbfragmentsharedelement
Class count: 27517
Instance count: 200528
Primitive array count: 147971
Object array count: 29487
Thread count: 19
Heap total bytes: 27495870
Bitmap count: 1
Bitmap total bytes: 622848
Large bitmap count: 0
Large bitmap total bytes: 0
Stats: LruCache[maxSize=3000,hits=112546,misses=194367,hitRate=36%]
RandomAccess[bytes=9557532,reads=194367,travel=76808102809,range=33363847,size=4
1840281]
Analysis duration: 3619051 ms

This leak is coming from AndroidX Transition and not Fragment. Please file an issue against that component.

Again, neither the toolbar or the leak is the issue. These are indications that something deeper is causing issues. Please just read what I am reporting and try the sample to experience the issue yourself.

Thanks, we'll take another look.

This has been fixed internally and will be available in the Fragment 1.7.0-beta01 release.

Thank you for your work.
Are the changes mentioned in #comment12 the internal fix? I have used the AndroidX snapshot build with this fix included and stumbled uponan occasional crash when navigating back using the gesture before the animation has finished. Also leakcanary is still complaining and dumping the same stack trace as posted before and using the back button in the toolbar sometimes leads to an undefined state where the second fragment is still shown but we should see the first fragment. I put a screen recording of all cases in the attachment.

FATAL EXCEPTION: main
Process: com.example.pbfragmentsharedelement, PID: 3103
java.lang.IllegalStateException: setCurrentPlayTimeMillis() called after animation has been started
	at androidx.transition.Transition$SeekController.setCurrentPlayTimeMillis(Transition.java:2778)
	at androidx.transition.FragmentTransitionSupport.setCurrentPlayTime(FragmentTransitionSupport.java:265)
	at androidx.fragment.app.DefaultSpecialEffectsController$TransitionEffect.onProgress(DefaultSpecialEffectsController.kt:798)
	at androidx.fragment.app.SpecialEffectsController.processProgress(SpecialEffectsController.kt:441)
	at androidx.fragment.app.FragmentManager$1.handleOnBackProgressed(FragmentManager.java:513)
	at androidx.activity.OnBackPressedDispatcher.onBackProgressed(OnBackPressedDispatcher.kt:255)
	at androidx.activity.OnBackPressedDispatcher.access$onBackProgressed(OnBackPressedDispatcher.kt:63)
	at androidx.activity.OnBackPressedDispatcher$2.invoke(OnBackPressedDispatcher.kt:129)
	at androidx.activity.OnBackPressedDispatcher$2.invoke(OnBackPressedDispatcher.kt:127)
	at androidx.activity.OnBackPressedDispatcher$Api34Impl$createOnBackAnimationCallback$1.onBackProgressed(OnBackPressedDispatcher.kt:388)
	at android.window.WindowOnBackInvokedDispatcher$OnBackInvokedCallbackWrapper.lambda$onBackStarted$0(WindowOnBackInvokedDispatcher.java:329)
	at android.window.WindowOnBackInvokedDispatcher$OnBackInvokedCallbackWrapper$$ExternalSyntheticLambda5.onProgressUpdate(Unknown Source:2)
	at android.window.BackProgressAnimator.updateProgressValue(BackProgressAnimator.java:161)
	at android.window.BackProgressAnimator.-$$Nest$mupdateProgressValue(Unknown Source:0)
	at android.window.BackProgressAnimator$1.setValue(BackProgressAnimator.java:61)
	at android.window.BackProgressAnimator$1.setValue(BackProgressAnimator.java:57)
	at com.android.internal.dynamicanimation.animation.DynamicAnimation.setPropertyValue(DynamicAnimation.java:731)
	at com.android.internal.dynamicanimation.animation.DynamicAnimation.doAnimationFrame(DynamicAnimation.java:687)
	at android.animation.AnimationHandler.doAnimationFrame(AnimationHandler.java:344)
	at android.animation.AnimationHandler.-$$Nest$mdoAnimationFrame(Unknown Source:0)
	at android.animation.AnimationHandler$1.doFrame(AnimationHandler.java:87)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1341)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1352)
	at android.view.Choreographer.doCallbacks(Choreographer.java:952)
	at android.view.Choreographer.doFrame(Choreographer.java:878)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1326)
	at android.os.Handler.handleCallback(Handler.java:958)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loopOnce(Looper.java:205)
	at android.os.Looper.loop(Looper.java:294)
	at android.app.ActivityThread.main(ActivityThread.java:8248)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)

If you used a snapshot from this morning it should be fixed. Are you doing the same thing in these 3 cases and seeing different results? Could you further explain how you get these 3 cases?

I did use the very latest snapshot version from this morning.

There are 2 different ways to trigger an issue and both issues trigger the same leak from #comment8.

The first issue, leading to the crash in the stacktrace in #comment14, is to:
Press the button to open the second fragment and interrupt the opening animation by performing the back gesture (without seeking).
Press the second fragment button again before the closing animation is finished and interrupt the opening animation again using the back gesture.

The second issue, to get into an undefined state, is to:
Press the button to open the second fragment.
Press the back button in the toolbar at the time the opening animation is finishing.
The timing has to be just right but the issue can be triggered fairly consistently.

Both issues can be experienced using the project attached to #comment5

The behaviour described in the bug fix from #comment12 returned with https://android-review.googlesource.com/c/platform/frameworks/support/+/3017979

I am closing this as I have not been able to reliably produce any of the mentioned errors with the latest library versions and this bug was about providing support for shared elements, which did. I did see a case where if you use system back to interrupt, that the transition didn't finish, but I couldn't reproduce that either.

If you can provide an app that reproduces this case reliably please file a new bug.

The last issue with predictive back I've come across was fixed in fragment-1.8.5. Thanks for the fixes.